Great point! The electrode arrays used in these studies are implanted in the participants' motor cortex in an area called the "hand knob". Neurons here are highly modulated during motor output, not only for the hand, but also for the rest of the body. This area is close to the output though, and abstract higher-level features like the "idea of an A" isn't likely to be decodable from neurons there. Future research is looking at exactly this, but will likely require recording in a different area.

For context, I did my PhD in the lab that did the work in this article.